As a piezoelectric element using a bulk wave which travels through a solid, a piezoelectric vibrator used as a source of clock signals for various electronic instruments or a piezoelectric filter used for sampling frequencies of communication instruments is known. Recently, the piezoelectric element has been used at a high frequency. Most of the piezoelectric elements employ thickness shear vibration or thickness longitudinal vibration as principal vibration, and a resonance frequency of the principal vibration is inversely proportional to a thickness of a piezoelectric plate. Therefore, the piezoelectric element is attempted to be used at a higher frequency by forming the piezoelectric plate with a thin film or the like.
The conventional piezoelectric element is disclosed in “Properties of Aluminum nitride thin films for piezoelectric transducers and microwave filter applications”, written by Marc-Alexander Dubois and Paul Murant, on pp. 3032-3034 of Applied Physics Letters in May 17, 1999.
As shown in conventional piezoelectric element 901 of FIG. 9, aluminum nitride thin film 91 is used as the piezoelectric plate, and exciting electrodes 92 are formed on its front and back. A dielectric film formed of a laminated layer of silicon nitride film 93 and silicon oxide film 94 is disposed at one surface of film 91. In addition, the whole piezoelectric vibrator is disposed on silicone substrate 95 so that very thin piezoelectric vibrator is treated easily.
In the conventional structure discussed above, an elastic constant of the piezoelectric film and the dielectric film and a size thereof are determined in a manner that a temperature coefficient of the resonance frequency of the piezoelectric vibrator becomes zero at an initial stage after deposition. However, in the conventional structure, long-term reliability for stress relaxation of each film is not considered. Therefore, according as time passes, a warp, namely a change in size, of the whole piezoelectric vibrator becomes larger than that at the initial stage because of a difference of the stress relaxation of the piezoelectric film and the dielectric film. As a result, this warp brings a change in resonance frequency.